1. Field of Invention
The present invention generally relates to a Thin Film Transistor (TFT) manufacture method, and more particularly, to a manufacture method of making an a-Si layer in the TFT. The method optimizes the electric characteristics of the TFT, and restrains the photo-leakage current generated when the a-Si layer in the TFT is irradiated by the light.
2. Description of Related Art
The fast growth of multimedia has mainly benefited from the great improvement in semiconductors and display apparatus. For the display, the Cathode Ray Tube (CRT) has continuously monopolized the display market in recent years due to its excellent display quality and advantage of being cost-effective. However, for the environment of the user using multiple terminals/display apparatus on a desktop, or from the environmental protection viewpoint and the trend of saving energy, CRTs have many problems due to deficiencies of space utilization and power consumption. As a result, CRTs cannot fulfill the requirements of the lighter, thinner, shorter, and smaller as well as low power consumption. Therefore, the Thin Film Transistor Liquid Crystal Display (TFT-LCD) having the advantages of the high display quality, good space utilization, low power consumption, and no radiation gradually has become the mainstream.
The general TFT-LCD mainly constitutes an LCD panel and a driving circuit. Wherein, the LCD panel comprises a TFT array substrate, a color filter substrate (C/F substrate), and a liquid crystal layer located in between these two substrates. The photo film plates such as the polarizer, the photo-intensity plate, and the diffusion plate are attached to the outside of two substrates. The driving circuit is generally bonded to the panel with the chip-bond manner. The Chip On Board (COB), the Chip On Glass (COG), and the Tape Automated Bonding (TAB) manners are generally used. Moreover, since the liquid crystal molecule cannot emit light itself, the light source with enough intensity must be provided for LCD to perform the display operation. For the reflection type panel, the light source generally comes from a front light module or an external light source. For the penetration type panel, the light source generally comes from a back light module.
The well known TFT can be generally divided into two different types: the a-Si layer TFT and the polycrystalline silicon TFT. Since the manufacture process of the a-Si TFT is simpler compared to the manufacture process of the polycrystalline silicon TFT, the technique of liquid crystal driven by the a-Si TFT array is still mainstream. However, there are some problems that exist in the a-Si TFT element itself, for example, the a-Si layer in the TFT will generate the photo leakage current Iphoto after it is irradiated by the front light source, the back light source or the external light source. The photo leakage current Iphoto not only impacts the performance of the TFT element itself, but also generates problems such as frame flicker or cross talk when the frame is displayed.
FIG. 1 schematically shows a flow chart for making the a-Si layer (channel layer) in a conventional TFT. Referring to FIG. 1, the a-Si layer (channel layer) in the conventional TFT is generally formed above the gate by using the Chemical Vapor Deposition (CVD) method. The manufacture of the a-Si layer is generally performed in three phases: (1) forming a first a-Si layer with a low deposition rate (LDR); (2) forming a second a-Si layer with a high deposition rate (HDR); and (3) performing the N+Mixed to the second a-Si layer to form a N+Mixed a-Si layer.
FIG. 2 lists the manufacture process parameters used in the first a-Si layer formed by using the LDR and the second a-Si layer formed by using the HDR in the prior art. Referring to FIG. 2, when the first a-Si layer is formed by using the LDR, the flux of SiH4 is 4400 sccm, the flux of H2 is 22000 sccm, and the flux ratio of H2/SiH4 is 5.0. Moreover, the operating pressure used to form the film of the first a-Si layer film is 1.1 mbar, and the power of the radio frequency (RF) is 140 W. Such a high flux of H2 can achieve the objective of repairing the defects. It means a large amount of the H2 links with the dangling bonds in the first a-Si layer, so that the number of the defects in the first a-Si layer can be reduced. In other words, the first a-Si layer with high quality film can be formed by controlling the H2/SiH4 flux ratio to stay at about 5.0 to perform the film forming with LDR.
Similarly, referring to FIG. 2, after the first a-Si layer is formed, the second a-Si layer is formed above the first a-Si layer by using the HDR. The flux of the Si4 is 5700 sccm, the flux of H2 is 5400 sccm, and the flux ratio of H2/SiH4 is 0.95. Moreover, the operating pressure used to form the film of the second a-Si layer film is 1.4 mbar, and the RF power is 250 W.
FIG. 3 lists the electric characteristics and averages of the conventional TFT. Referring to FIG. 3, the channel layer in the conventional TFT is made by using the manufacture process conditions listed in FIG. 2. The average of the on current Ion of the conventional TFT is about 6.572 μA, the average of the off current Ioff is about 4.7278 pA, the average of the threshold voltage Vth is about 3.3496V, and the average of the electronic migration rate μfe is about 0.5588 cm2/v.s.
The flux ratio of H2/SiH4 is controlled to stay at about 5.0 when the first a-Si layer is made by using the LDR in the prior art. The high flux of H2 is applied to the first a-Si layer to repair the defects, and further to have the off current Ioff reduce to about 4.7278 pA (below 5 pA). However, when evaluating the performance of the a-Si TFT element itself, besides considering the parameters such as the on current Ion, the off current Ioff, the threshold voltage Vth, and the electronic migration rate μfe, the photo leakage current Iphoto of the element is also served as a factor of the evaluation. The on current Ion, the off current Ioff, the threshold voltage Vth, and the electronic migration rate μfe of the conventional TFT are all in the adequate range. However, the TFT generates photo leakage current Iphoto after it is irradiated by the front light source, the back light source or the external light source. The photo leakage current Iphoto generated usually is about 1E–10 Amp, and it significantly deteriorates the display quality.